Gelatin is commonly used as a vehicle in photographic materials. Quite often, numerous layers in photographic elements, such as light-sensitive silver halide layer, filter layers, backing layers, antihalation layers, and overcoat layers, use gelatin as the primary binder.
Photographic elements are generally processed in aqueous processing solutions. Such processing results in excessive swelling and loss of strength of the gelatin binder unless the gelatin has been hardened. If the processing solution is maintained at a warm temperature, e.g., 40.degree. C., the gelatin binder can dissolve, causing the layers of the element to disintegrate, unless the gelatin has been hardened.
A number of compounds have been used to harden gelatin. These include formaldehyde and free aldehydes as described in U.S. Pat. No. 3,232,764, active esters as described in U.S. Pat. No. 3,542,558, epoxy compounds as described in U.S. Pat. No. 3,047,394, aziridines as described in U.S. Pat. No. 2,950,197, and others well-known in the art. These compounds harden, or crosslink, gelatin, thus increasing its mechanical strength and reducing the swellability and solubility of the gelatin in aqueous processing solutions.
The above-described hardening compounds are, however, subject to a number of disadvantages. Some take an exceedingly long time after being cast and dried to give the desired degree of gelatin hardening, requiring elements containing gelatin hardened with those compounds to be aged for an extended time while the hardening process is completed. In other words, these hardeners exhibit after-hardening. Additionally, a number of compounds exhibit adverse effects on the photographic properties of elements in which they are used. Such adverse effects can include an increase in fog or a reduction in light sensitivity or photographic speed.
In order to avoid the time and expense of keeping large quantities of gelatin-containing photographic elements for long periods while the hardening process is completed, a number of so-called fast-acting hardeners have been described in the art. For example, U.S. Pat. No. 4,063,952 describes a carbamoyl pyridinium hardening compound in which the pyridine ring carries a sulfoalkyl substituent. European patent application No. 162,308 describes a chloroformamidinium hardener, and U.S. Pat. No. 4,612,280 describes an N-succinimidyloxyformamidinium hardener. These hardeners, however, suffer from a number of problems. Some of these hardeners exhibit after-hardening. Others have severe handling difficulties due to their hygroscopic nature, or poor hydrolytic stability, or both. Also, some of these hardeners adversely affect either the physical properties of photographic elements in which they are used (e.g., tackiness), or their sensitometric properties (e.g., speed loss). These problems are often aggravated by the fact that relatively large amounts of hardener are often required to achieve the desired hardening effect, leading to relatively large amounts of sometimes deleterious by-products.
It is therefore desirable to provide a hardener that effectively hardens gelatin in a short period of time, while substantially avoiding or reducing many of the above-described adverse effects. It is toward that end that the present invention is directed.